This is a comparative study of: 1) multiple sclerosis prevalence among American-born migrants to and Washington-born natives of Seattle, Washington (a high-risk area), and American-born migrants to and California-born natives of Southern California (a low-risk area) to assess environmental factors in the pathogenesis and prevention of multiple sclerosis. 2) MS prevalence among foreign-born and American- born Japanese in Seattle and in Southern California to help determine if genetic factors are responsible for the unexpectedly low rate of MS reported in Japan. 3) duration and severity of MS among native-born Californians in Los Angeles and native-born Washingtonians in Seattle and among migrants from high and from low-risk areas to these two areas in order to investigate if a relationship exists between environmental factors and duration and severity of disease. Population information will be derived from the 1970 census and cases identified through the cooperation of the local MS societies, medical societies, neurologists, internists, and hospitals. Characteristics of patients will be ascertained by interview of the MS patients and their physicians. Diagnostic comparability among subgroups will be confirmed by examination by a single neurologist of a 20 percent probability sample of the MS patients. The findings of this study are expected to provide information about the nature of possible causal factors, etiologic mechanisms and protection in this chronic neurologic disease.